Semiconductor devices using a nitride semiconductor can improve the trade-off relationship between the breakdown voltage and the ON resistance to achieve a decreased ON resistance and an increased breakdown voltage because of the excellent material characteristics thereof. Examples of the structure of such a nitride semiconductor device include a field effect transistor using a hetero-structure of AlGaN and GaN. When it is attempted to achieve normally OFF operation in the structure, a recess gate structure is used in which an AlGaN layer below a gate electrode is made thinner than the other portions by etching. Further improvement is required in order to obtain stable characteristics in such a nitride semiconductor device.